1. Field of the Invention
The present invention relates generally to a multilayered ceramic RF (radio frequency) device, and in particular but not exclusively, to a multilayered ceramic RF device used in high-frequency radio equipment such as a cellular telephone.
2. Description of the Related Art
Recently multilayered ceramic RF devices are attracting much attention for their ability to contribute greatly to the size reduction of high-frequency radio equipment such as a cellular telephone.
FIG. 15 is a block diagram showing an example of an RF circuit used in a cellular telephone. A duplexer 22 is formed of a transmitting filter and a receiving-filter. In such an RF circuit as show in FIG. 15, a transmitting signal amplified by a power amplifier 21 passes through a low-pass filter 20 and the transmitting filter in the duplexer 22 and is transmitted from an antenna 24. A band-pass filter may be used instead of the low-pass filter 20. A signal received by the antenna 24 is input to a low-noise amplifier 28 via the receiving filter in the duplexer 22 and a band-pass filter 26. The signal is amplified by the low-noise amplifier 28 and, after that, the signal is subjected to a frequency conversion and a signal processing. While the low-pass filter 20 and the duplexer 22 can be constituted from multilayered filters or the like, the band-pass filter 26 is normally constituted from a SAW filter. The power amplifier 21 and the low-noise amplifier 28 are made by using semiconductor elements with excellent radio frequency characteristics.
Now an example of the multilayered ceramic RF device of the prior art will be described below with reference to FIG. 16 and FIG. 17.
FIG. 16 is a sectional view of a multilayered ceramic RF device 100 of the prior art which constitutes a part of the RF circuit shown in FIG. 15. In the multilayered ceramic RF device 100 of the prior art, electrode patterns 102 which constitute the RF circuit are formed in a low temperature-cofired multilayered ceramic body 101. The electrode patterns 102 are electrically connected to each other by means of via holes 103. Chip components 105 such as chip resistors, chip capacitors, chip inductors and packaged semiconductor elements are formed on the surface of the low temperature-cofired multilayered ceramic body 101, and are shielded by a metal cap 107.
The operation of the multilayered ceramic RF device 100 of the prior art constituted as described above will be described below.
The electrode patterns 102 form inner layer capacitors and inner layer inductors in the low temperature-cofired multilayered ceramic body 101 as well as providing electrical connection between the plurality of chip components 105. These components collectively form the RF circuit and serve as a multilayered ceramic RF device such as, for example, a multilayer RF switch.
FIG. 17 schematically shows an example of the constitution of an RF device 120 of the prior art used in a cellular telephone that has the RF circuit as shown in FIG. 15. As shown in FIG. 17, the RF device 120 of the prior art has been constituted from separate components such as an multilayered filter 110, a SAW filter 112 covered with a ceramic package 111, and the mutilayered ceramic RF device 100 formed of a multilayered ceramic body having an RF switching circuit 114, which are independent from each other. Thus the RF device 120 of the prior art shown in FIG. 17 has been made by mounting the multilayered filter 110, the SAW filter 112 covered with a ceramic package 111, and the multilayered ceramic RF device 100 that are independent from each other on a printed circuit board and connecting the components by soldering or using micro strip lines.
In the constitution of the RF device 120 of the prior art, however, there has been such a problem that the use of soldering lands or running the micro strip lines on the printed circuit board causes unmatched impedance and/or an increase in impedance loss. Also because connection of the components is carried out in the final packaging stage of the production process, even when the components have been certified for satisfactory radio frequency characteristics before connection, there occur variations in the high-frequency characteristics of the components due to unmatched impedance in junctions or the like after the components have been connected in the final packaging stage. As a result, it has been difficult to produce the RF device 120 of the prior art with excellent radio frequency characteristics and excellent reproducibility.
Also in the multilayered ceramic RF device 100 of the prior art shown in FIG. 16, since a bare semiconductor chip (i.e., a chip that is not molded) and a SAW filter need to be sealed, these components cannot be used in the device 100. This is because the metal cap 107 of the prior art is used for the purpose of merely providing an electromagnetic shielding and does not have the sealing function. Therefore, it is necessary to use the semiconductor element and the SAW filter that are individually sealed, which makes it difficult to reduce the device size and leads to a complicated manufacturing process.
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an objective of the present invention to provide a multilayered ceramic RF device that has excellent radio frequency characteristics and high reliability.
Another object of the present invention is to provide a multilayered ceramic RF device that has high performance, and is small in size and in profile and easy to produce.
In accomplishing the above and other objectives, a multilayered ceramic RF device of the present invention having at least one radio frequency filter, includes a low temperature-cofired multilayered ceramic body having a plurality of ceramic layers laminated one upon another and fired together, the low temperature-cofired multilayered ceramic body also having a first electrode pattern formed therein and a second electrode pattern formed thereon. The first and second electrode patterns are electrically connected to-one another through a-via hole. A bare semiconductor chip (i.e., a chip that is not molded) is mounted on the low temperature-cofired multilayered ceramic body with a face down bonding, and the bare semiconductor chip is coated with a sealing resin. The least one radio frequency filter is a multilayered filter formed in the low temperature-cofired multilayered ceramic body, and the multilayered filter includes a part of the first and second electrode patterns.
In the multilayered ceramic RF device of the present invention, the electrode patterns formed on the surface and inside of the low temperature-cofired multilayered ceramic body are electrically connected with each other by via holes (holes that penetrate the ceramic layer and are filled with, for example, Ag or Cu), and the radio frequency filter is a multilayered filter formed inside of the multilayered ceramic body while including a part of the electrode patterns. Thus the connection between the multilayered filters, between the multilayered filter and another radio frequency filter or between the multilayered filter and a bare semiconductor chip can be made with very short wiring a distances by using the via holes or the like that are formed inside the multilayered ceramic body. As a result, unmatched impedance and impedance loss can be decreased, ripple in the pass band of the radio frequency filter can be prevented and proper performance of the filter can be realized, compared to the RF device of the prior art having components such as the radio frequency filter and the multilayered filter individually mounted on the printed circuit board. Therefore, the multilayered ceramic RF device that has excellent high-frequency characteristics and high reliability can be provided. Also the device can be made small in size and in profile with a reduced number of components, and can be produced in a simplified process.
The multilayered filter may be formed as a distributed constant multilayered filter that has a strip line resonator, and the strip line resonator can be formed to include a strip line resonator electrode that is formed as the part of the electrode patterns.
The multilayered filter may be formed as a lumped constant multilayered filter including a capacitor electrode and an inductor electrode, and the capacitor electrode and the inductor electrode can be formed as the part of the electrode patterns.
In the case where the multilayered ceramic RF device includes two or more radio frequency filters, at least one of the two or more radio frequency filters may be a SAW filter.
The multilayered ceramic RF device can be sealed easily, provided that the multilayered ceramic body has a cavity formed therein substantially at a center thereof, and the bare semiconductor chip is mounted at a bottom of the cavity in which the sealing resin is filled so as to cover the bare semiconductor chip. Also, because the sealing resin can be prevented from spreading to the side face of the multilayered ceramic body, a defect in production such as when the sealing resin covers the side electrode can be prevented from occurring when the side electrodes are provided on the multilayered ceramic body.
Such a constitution may also be employed as a plurality of ceramic layers including a first ceramic layer that has a first relative dielectric constant and a second ceramic layer that has a second relative dielectric constant that is different from the first relative dielectric constant. This constitution makes it possible to provide an element (an element formed to have the part of the electrode patterns in the multilayered ceramic body) that is suited to include the first ceramic layer which has the first relative dielectric constant formed on the first ceramic layer, and provide an element that is suited to include the second ceramic layer which has the second relative dielectric constant formed on the second ceramic layer. Thus the multilayered ceramic RF device that has high reliability can be provided.
Preferably, the plurality of ceramic layers include a top layer, a bottom layer, and an intermediate layer sandwiched between the top layer and the bottom layer, and the intermediate layer is formed of the first ceramic layer with a first relative dielectric constant. Both the top layer and the bottom layer are formed of the second ceramic layer with a second relative dielectric constant. This makes it possible to reduce the warp of the ceramic layers during firing. Moreover, since the elements formed to have the part of the electrode patterns in the multilayered ceramic body can be formed in the ceramic layer that has a relative dielectric constant most suitable for the element, the multilayered ceramic RF device having further higher reliability can be provided. The first relative dielectric constant is preferably greater than or equal to 10, and the second relative dielectric constant is preferably smaller than 10.
In the case where the-bare semiconductor chip is connected to the multilayered filter, the bare semiconductor chip is preferably placed over the multilayered filter. That is, when the area of the electrode patterns included in the multilayered filter is substantially equal to the sectional area of the bare semiconductor chip (area of the section perpendicular to the laminating direction of the ceramic layers included in the multilayered filter) and the bare semiconductor chip is arranged so as to overlap the multilayered filter, the bare semiconductor chip and the multilayered filter can be connected with the shortest wiring length. As a result, the size of the multilayered ceramic RF device can be made even smaller.
According to the present invention, the multilayered ceramic RF device having at least one radio frequency filter, includes a low temperature-cofired multilayered ceramic body having a plurality of ceramic layers laminated one upon another and fired together, and having a cavity therein. A first electrode pattern is formed therein and a second electrode pattern is formed thereon, the first and second electrode patterns being electrically connected to one another through a via hole. At least one of a bare semiconductor chip and a SAW filter is mounted at a bottom of the cavity, and a sealing metal cover seals the cavity hermetically. The at least one radio frequency filter is a multilayered filter formed in the low temperature-cofired multilayered ceramic body, and the multilayered filter includes a part of the first and second electrode patterns. Since the low temperature-cofired multilayered ceramic body has the cavity on the top surface thereof with at least one of the bare semiconductor chip and the SAW filter mounted at the bottom of the cavity and the cavity is hermetically sealed with a sealing metal cover, the at least one of the bare semiconductor chip and the SAW filter can be incorporated in the low temperature-cofired multilayered ceramic body. As a result, a more compact multilayered ceramic RF device can be made and hermetic sealing can be easily achieved.
The multilayered filter may be formed as a distributed constant multilayered filter that has a strip line resonator, and the strip line resonator can be formed to include a strip line resonator electrode that is formed as the part of the electrode patterns. In case the strip line resonator electrode is formed in the multilayered ceramic body in a region other than where the cavity is formed on the top surface, sufficient thickness of the strip line resonator electrode can be secured without increasing the device height.
When the strip line resonator electrode is formed in the multilayered ceramic body, it is preferable to form the plurality of ceramic layers included in the multilayered ceramic body from the intermediate layer having a low dielectric constant (first relative dielectric constant) and the top layer and the bottom layer that have a high dielectric constant (second relative dielectric constant), and provide the strip line resonator electrode on the bottom layer having the high dielectric constant. This makes it possible to form the strip line resonator electrode of smaller size.
The multilayered ceramic RF device may also have a radio frequency switching circuit that includes a capacitor electrode and an inductor electrode. The electrode patterns formed in the low temperature-cofired multilayered ceramic body may include at least one of the capacitor electrode and the inductor electrode. When the multilayered ceramic RF device additionally has the radio frequency switching circuit such as the above, the multilayered ceramic RF device having more versatile functions can be provided.
If the multilayered ceramic RF device is used for both of a W-CDMA (wideband code division multiple access) and a GSM (global system for mobile communication), it is preferable that the multilayered filter is the transmitting filter of the WCDMA, the SAW filter is a receiving filter for the W-CDMA, and the RF switching circuit is a switching duplexer for the GSM.